Atomic oscillator

Abstract

An atomic oscillator (10) comprises: a gas cell (8) in which alkali metal atoms are sealed; a photodetector (9) which detects light transmitted through the gas cell (8) and converts the light into an electric signal (SLI); a control circuit (1) which includes a high-frequency oscillator (31) using an FBAR, inputs the signal (SLI), and outputs a feedback signal (SFB); and a laser light source (6) which outputs light modulated by the feedback signal (SFB) and introduces the light into the gas cell (8). The control circuit (1) further comprises a variable capacitor (32) that adjusts the oscillation frequency of the high-frequency oscillator (31), a frequency converter (41) that outputs a high-frequency adjustment signal (Stune), and a frequency mixer (42) that mixes a signal (SOUT) output by the high-frequency oscillator (31) with the high-frequency adjustment signal (Stune) and outputs a signal having a frequency with which the resonance frequency of the gas cell (8) is synchronized.

Description

Atomic oscillator

[0001] The present invention relates to an atomic oscillator.

[0002] Atomic oscillators include atomic clocks, as well as small atomic frequency standards used in inertial sensors and magnetic sensors that utilize atomic resonance, such as those that utilize CPT (Coherent Population Trapping, quantum interference effect) resonance. An atomic oscillator using CPT (hereinafter referred to as an atomic oscillator) includes a gas cell (atomic resonance system) in which an alkali metal atom is enclosed together with a buffer gas, a laser light source that inputs light into the atomic resonance system, and a high-frequency (microwave) oscillation circuit. The high-frequency oscillation circuit synchronizes (locks) the signal output from the atomic resonance system (resonance signal) with the signal output by the high-frequency oscillation circuit to output a stable frequency. In atomic oscillators, a chip-scale atomic clock (Chip-Scale Atomic Clock; CSAC) has been developed that is miniaturized to the chip level and incorporates a gas cell with one side miniaturized to about several millimeters.

[0003] As an example, as shown in FIG. 9, the atomic oscillator 110 includes a laser light source 6, a gas cell 8, a photodetector 9 that converts the light transmitted through the gas cell 8 into an electrical signal, and a control circuit 101 that includes a high-frequency oscillation circuit 103. The control circuit 101 inputs the signal S LI output by the photodetector 9 and modulates the signal S OUT output by the high-frequency oscillation circuit 103 to generate a feedback signal S FB In FIG. 9, the control circuit 101 further incorporates a local oscillator 21 that oscillates a control signal S cnt in the kHz band, a discriminator 2 that generates an error signal S err and a modulator 5 that modulates the signal S OUT output by the high-frequency oscillation circuit 103 based on the control signal S cnt and outputs a feedback signal S FB The laser light source 6 outputs light modulated based on the feedback signal S FB In FIG. 9, solid arrows schematically represent alternating electrical signals, thick arrows represent direct current, and hatched arrows represent light.

[0004] The high-frequency oscillation circuit 103 is equipped with a crystal oscillator 131 and generates a high-frequency signal S in the GHz band, which is half the frequency of the transition frequency of alkali metal atoms in the gas cell 8. OUT It outputs a high-frequency signal S. In contrast, the maximum oscillation frequency of a crystal oscillator is about 300 MHz. Therefore, the high-frequency oscillation circuit 103 is equipped with a frequency divider 132, a phase comparator (PC) 133, a loop filter (LF) 134, and a voltage-controlled oscillator (VCO) 135, and performs frequency multiplication processing using a phase-locked loop (PLL) to produce a high-frequency signal S. OUT To obtain.

[0005] Due to its structure, the crystal oscillator 131 is difficult to manufacture on-chip, unlike other elements of the control circuit 101 such as the local oscillator 21, which is composed of an RC oscillator circuit or an LC oscillator circuit, and this also limits the miniaturization of atomic clocks. On the other hand, in recent years, piezoelectric thin-film resonators (Film Bulk Acoustic Resonators; FBARs) that can be manufactured on-chip and output high-frequency signals in the GHz band have been developed (for example, Non-Patent Documents 1-3), and atomic clocks that apply these to high-frequency oscillators have been proposed (Non-Patent Document 4).

[0006] M. Hara, T. Yokoyama, T. Sakashita, S. Taniguchi, M. Iwaki, T. Nishihara, M. Ueda, Y. Satoh, “Super-high-frequency band filters configured with air-gap-type thin-film bulk acoustic resonators”, Japanese Journal of Applied Physics, Volume 49, Number 7S, pp.07HD13, 2010M. Ramezani, M. Ghatge, R. Tabrizian, “High-Q silicon fin bulk acoustic resonators for signal processing beyond the UHF”, Proceedings of 2017 IEEE International Electron Devices Meeting (IEDM), pp.40.1.1-4, 2017R. Vetury, A. Kochhar, J. Leathersich, C. Moe, M. Winters, J. Shealy, R. H. Olsson, “A Manufacturable AlScN Periodically Polarized Piezoelectric Film Bulk Acoustic Wave Resonator (AlScN P3F BAW) Operating in Overtone Mode at X and Ku Band”, Proceedings of 2023 IEEE / MTT-S International Microwave Symposium - IMS 2023, pp.891-894, 2023M Hara, Y Yano, M Kajita, H Nishino, Y Ibata, M Toda, S Hara, A Kasamatsu, H Ito, T Ono, T Ido, “Microwave oscillator using piezoelectric thin-film resonator aiming for ultraminiaturization of atomic clock”, Review of Scientific Instruments, Vpl.89(10), pp.105002, 2018.

[0007] Crystal oscillators have low manufacturing variations due to high-precision machining, and the oscillation frequency tolerance is generally around ± tens of ppm, and around ± 10 ppm in applications requiring particularly high precision. Therefore, the frequency of high-frequency signals can be precisely controlled by using a crystal oscillator. In contrast, high-frequency oscillators using FBARs are greatly affected by manufacturing variations such as the thickness of the piezoelectric film, and the oscillation frequency tolerance is generally on the order of 0.1% to %. Although the frequency can be adjusted by a variable capacitor connected to the FBAR (Non-Patent Literature 4), there are limitations as the Q value of the oscillation deteriorates when the variable range is expanded, and it is insufficient to cover manufacturing variations. As a result, high-frequency oscillators that can be used in atomic oscillators have a low manufacturing yield, and the cost of atomic oscillators is high.

[0008] This invention has been made in view of the above-mentioned problems, and aims to provide an atomic oscillator that can suppress high costs, be further miniaturized, and be manufactured on-chip.

[0009] The atomic oscillator according to the present invention generates a resonant frequency due to CPT resonance and comprises a gas cell containing alkali metal atoms, a photodetector that detects light transmitted through the gas cell and converts it into an electrical signal, a control circuit equipped with a high-frequency oscillator that generates a feedback signal from the electrical signal and the signal output by the high-frequency oscillator, and a laser light source that outputs light modulated by the feedback signal and introduces it into the gas cell. The atomic oscillator further comprises a variable capacitor that adjusts the oscillation frequency of the high-frequency oscillator using the signal output by the photodetector, a high-frequency adjustment oscillator circuit that outputs a high-frequency adjustment signal, and a frequency mixer that mixes the signal output by the high-frequency oscillator or a modulated signal of the signal with the high-frequency adjustment signal and outputs a signal at a frequency that synchronizes the resonant frequency of the gas cell, wherein the maximum frequency that the high-frequency adjustment oscillator circuit can output is greater than the variable range of the oscillation frequency of the high-frequency oscillator by the variable capacitor.

[0010] With this configuration, even if the atomic oscillator is equipped with a high-frequency oscillator with a relatively large tolerance for the oscillation frequency, the frequency can be adjusted to synchronize the resonance frequency.

[0011] According to the present invention, an atomic oscillator can be obtained that is even smaller, incorporates a high-frequency oscillator that can be integrated with a circuit system, suppresses cost increases, and can be manufactured on-chip.

[0012] This is a block diagram illustrating the structure of an atomic oscillator according to the first embodiment of the present invention. This is a schematic diagram illustrating the frequency adjustment of a high-frequency signal in an atomic oscillator according to the present invention. This is a schematic diagram illustrating the frequency adjustment of a high-frequency signal in an atomic oscillator according to the present invention. This is a schematic diagram illustrating the frequency adjustment of a high-frequency signal in an atomic oscillator according to the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to a modified example of the first embodiment of the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to the second embodiment of the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to a modified example of the second embodiment of the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to a modified example of the second embodiment of the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to the third embodiment of the present invention. This is a block diagram illustrating the structure of an atomic oscillator according to a modified example of the third embodiment of the present invention. This is a block diagram illustrating the structure of an example of an atomic oscillator equipped with a crystal oscillator.

[0013] Embodiments for implementing the atomic oscillator according to the present invention will be described with reference to the figures. Elements with the same structure and characteristics are denoted by the same reference numerals, and their descriptions are omitted.

[0014] [First Embodiment] As shown in Figure 1, the atomic oscillator 10 according to the first embodiment of the present invention comprises a gas cell 8 containing alkali metal atoms and a transmitted light intensity signal S, which is an electrical signal obtained by detecting light transmitted through the gas cell 8. LI It includes a photodetector 9 that converts to a signal S and a high-frequency oscillator 31, LI Input the feedback signal S FB A control circuit 1 outputs a feedback signal S. FBThe atomic oscillator 10 includes a laser light source 6 that outputs modulated light based on and introduces it into the gas cell 8. Furthermore, the atomic oscillator 10 includes an optical system 7 that converts the light output by the laser light source 6. The atomic oscillator 10 may also include, if necessary, a solenoid coil wound around the gas cell 8 to apply a magnetic field in the direction of the optical axis in order to control the bias magnetic field of the gas cell 8, and a magnetic shield to block external magnetic fields such as the Earth's magnetic field (not shown). The atomic oscillator 10 generates CPT resonance in the gas cell 8 with the light introduced by the laser light source 6, modulates an electrical signal tuned to the resonant frequency and feeds it back to the laser light source 6 as a feedback signal, and also outputs it externally as a frequency standard signal, so that it can be used as an atomic clock. In Figure 1 and Figures 3 to 8 below, solid arrows schematically represent AC electrical signals, thick arrows represent DC currents, and hatched arrows represent light. The following describes each element in detail.

[0015] (Laser light source) The laser light source 6 is a light source that emits light that induces an optical transition from the ground state to the excited state in alkali metal atoms enclosed in the gas cell 8. For example, 87 For Rb's D1 line, it emits light with a wavelength of 795 nm. Furthermore, the difference frequency of this light is the transition frequency f of alkali metal atoms. clock Two sidebands are generated to match this. Note that the transition frequency f clock This corresponds to the energy difference between the two ground states of the alkali metal atom. Therefore, the laser light source 6 receives the feedback signal S output by the control circuit 1. FB Therefore, frequency f clock It is modulated by 2. The laser light source 6 consists of a laser element 63, a current source 61 and a Bias-T circuit 62 for supplying a current to the laser element 63 to output light of the aforementioned wavelength. The laser element 63 is preferably a semiconductor laser that can irradiate light of the aforementioned wavelength and is suitable for miniaturizing the atomic oscillator 10, and a VCSEL (Vertical Cavity Surface Emitting Laser) is particularly preferred. The Bias-T circuit 62 receives a feedback signal S output by the control circuit 1 to the DC current output by the current source 61.FB This is superimposed. This feedback signal S FB A current with the superimposed current is supplied to the laser element 63. The Bias-T circuit 62 can be manufactured on the same substrate as the control circuit 1.

[0016] (Optical System) The optical system 7 is positioned between the laser element 63 and the gas cell 8, and for example, it is equipped with a collimator 71, a polarizer 72, and a quarter-wave plate (λ / 4 plate) 73 arranged in order from the light incident side (the side of the laser element 63). The optical system 7 converts the light output by the laser element 63 into parallel light with a diameter corresponding to the gas cell 8, and further converts it into circularly polarized light before introducing it into the gas cell 8. Alternatively, the optical system 7 may be equipped with a polarizer whose absorption axis is not parallel to that of the polarizer 72, instead of the quarter-wave plate 73, positioned between the gas cell 8 and the photodetector 9. In this case, linearly polarized light is introduced into the gas cell 8.

[0017] (Gas Cell) The gas cell 8 is an atomic resonance system in the atomic oscillator 10, and consists of alkali metal atoms in a gaseous state and a buffer gas sealed in a light-transmitting cell. The gas cell 8 receives light from the laser light source 6, absorbs a portion of it with the alkali metal atoms, and directs the transmitted light into the photodetector 9. The gas cell 8 preferably has a structure that can be manufactured on the same substrate as the control circuit 1 using MEMS (Micro Electro Mechanical Systems) technology, and known structures can be applied. As an example, the gas cell 8 can be a transmissive gas cell in which a silicon substrate 1 to several mm thick with through-holes of several mm in diameter is sandwiched between two glass plates. Then, the laser element 63 is positioned opposite one of the glass plates of the gas cell 8, with the optical system 7 in between, and the photodetector 9 is positioned opposite the other glass plate. Alternatively, the gas cell 8 may be a reflective gas cell in which the holes in the silicon substrate are sandwiched between glass plates and a light-reflecting film, with the glass plate side serving as the light inlet and outlet. In this case, the optical system 7 is equipped with a circulator on the light emission side (the side closest to the gas cell 8), and the photodetector 9 is positioned so that the light that has passed through the gas cell 8 and been emitted is incident on it.

[0018] The alkali metal atoms sealed in gas cell 8 are cesium ( 133Cs) and rubidium ( 85 Rb, 87 Rubidium is preferred for further miniaturization of the gas cell 8, as Rb) is applied. When alkali metal atoms in the gas cell 8 are irradiated with light of a specific wavelength, they absorb it and transition from the ground state to an excited state. Furthermore, the difference frequency of this specific wavelength of light is the transition frequency f of the alkali metal atoms. clock If two sidebands matching this frequency are present, the light transmitted through the alkali metal atoms without being absorbed will have a transition frequency f clock It becomes stronger at the transition frequency f. clock teeth, 87 Rb: approximately 6.8 GHz, 85 Rb: Approximately 3.0GHz, 133 Cs: Approximately 9.2 GHz. Buffer gas is sealed in to suppress the effects of shortened relaxation times of excitation level lifetimes due to collisions of alkali metal atoms with the cell walls, and inert gases such as nitrogen or argon are used.

[0019] (Photodetector) The photodetector 9 converts the light transmitted through the gas cell 8 into an electrical signal (transmitted light intensity signal S). LI It is converted to and output as a resonance signal. The photodetector 9 is a photodiode corresponding to the wavelength range of light output by the laser light source 6, and a highly sensitive one is preferable. Furthermore, by making the response speed of the photodetector 9 sufficiently fast, the frequency f clock By extracting nearby microwaves, a low-noise signal can be obtained.

[0020] (Control circuit) Control circuit 1 receives a resonance signal S from the photodetector 9. LI This input provides a stable frequency to the outside, as well as the modulation signal (feedback signal) S of the laser light source 6. FB It outputs the following. The control circuit 1 can be a known configuration used in atomic oscillators using CPTs, and as an example, it comprises a discriminator 2, a high-frequency oscillation circuit 3, and a modulator 5. Furthermore, in this embodiment, the control circuit 1 comprises a frequency converter 41 and a frequency mixer 42. The discriminator 2 is a lock-in amplifier and outputs the control signal S oscillated by the built-in local oscillator 21. cnt and resonance signal S LI Error signal S errThe high-frequency oscillation circuit 3 generates the error signal S. err Input this signal S err The oscillation is locked and a stable frequency is provided to the outside, and the modulated signal S of the laser light source 6 FB The underlying high-frequency signal S OUT The modulator 5 outputs the high-frequency signal output by the high-frequency oscillation circuit 3, and the control signal S from the local oscillator 21. cnt The signal is modulated as a modulation signal (frequency modulation (FM) and / or phase modulation (PM)), or the control signal S cnt Mixed with the feedback signal S FB The output is as follows: Control circuit 1 can be manufactured on the same substrate using a CMOS (Complementary Metal Oxide Semiconductor) process or the like, and can be implemented as an on-chip structure.

[0021] The high-frequency oscillation circuit 3 comprises a high-frequency oscillator 31 using a piezoelectric thin-film resonator (FBAR) and a variable capacitor 32 for adjusting its oscillation frequency (see Non-Patent Document 4). In Figure 1 and Figures 3 to 8 below, the high-frequency oscillation circuit 3 is simply represented by the graphic symbol for an AC signal source (high-frequency oscillator 31) and the graphic symbol for a variable capacitor (variable capacitor 32). The high-frequency oscillator 31 is f clock It has an oscillation frequency near / 2, i.e., in the GHz band. For example, alkali metal atoms in gas cell 8 87 If Rb is present, the oscillation frequency of the high-frequency oscillator 31 is approximately 3.4 GHz. The variable capacitor 32 controls the error signal S. err Using this, the oscillation frequency of the high-frequency oscillator 31 is adjusted so that it synchronizes (locks) with the resonance frequency of the gas cell 8. The variable capacitor 32 further continuously adjusts the oscillation frequency in response to minute fluctuations in the resonance frequency over the long term, i.e., in the steady state after the atomic oscillator 10 has been synchronized. On the other hand, the variable range of the oscillation frequency by the variable capacitor 32 (adjustment range Δf tune (Maximum value of) Δf tune_max As the value increases, the phase noise of the oscillation of the high-frequency oscillator 31 increases, and the Q value deteriorates. Therefore, the variable width Δf of the variable capacitor 32 tune_max This is the unadjusted oscillation frequency f of the high-frequency oscillator 31.init (or f clock It is preferable to set it to 2000 ppm (several MHz) or less, and it is preferable to make it as small as possible within a range that can accommodate fluctuations in the steady state resonance frequency.

[0022] Here, the high-frequency oscillator 31 generally has a tolerance Δf of oscillation frequency due to manufacturing variations in the FBAR. tol The difference is large, ranging from 0.3% to 3% (tens to hundreds of MHz), and with adjustment (fine-tuning) using the variable capacitor 32, f clock There are cases where it is not possible to output / 2 (f init +Δf tune_max <f clock / 2, f init -Δf tune_max > f clock / 2). Therefore, the control circuit 1 uses the frequency converter 41 and the frequency mixer 42 to set the frequency f clock Obtain a signal of / 2.

[0023] The frequency converter 41, together with the local oscillator 21 of the discriminator 2, constitutes the high-frequency adjustment oscillation circuit 4. The frequency converter 41 receives the control signal S oscillated by the local oscillator 21. cnt Convert this to produce a signal for coarse adjustment (high-frequency adjustment signal) S tune The frequency converter 41 generates the frequency f tune_c Sweep signal S tune It can output the following. The frequency converter 41 is a circuit that performs frequency division / multiplication using, for example, a phase-locked loop (PLL). Signal S tune frequency f tune_c The maximum value is the tolerance Δf of the oscillation frequency of the high-frequency oscillator 31. tol and the variable range Δf of the oscillation frequency due to the variable capacitor 32 tune_max The difference (Δf tol -Δf tune_max ) and preferably Δf tol That concludes the explanation. Variable width Δf by variable capacitor 32. tune_max This is the tolerance Δf of the oscillation frequency of the high-frequency oscillator 31. tol Because it is designed to be smaller than, frequency f tune_c The maximum value is the variable width Δf of the variable capacitor 32.tune_max is greater than. Also, the frequency converter 41 sweeps the frequency f tune_max in steps of Δf or less. The frequency mixer 42 mixes the high-frequency signal S tune_c with the high-frequency adjustment signal S OUT to output a signal S tune having the sum or difference frequency of the two. o1

[0024] (Frequency stabilization operation of the atomic oscillator) The frequency stabilization operation of the atomic oscillator according to the present embodiment will be described below with reference to FIG. 1.

[0025] The laser light source 6 is modulated at a frequency of f / 2 such that the beat frequency matches the transition frequency f of the alkali metal atom based on the feedback signal S FB from the control circuit 1, and irradiates light having two sidebands of ν0 ± f / 2. This light becomes circularly polarized via the optical system 7 and is introduced into the gas cell 8. The light transmitted through the gas cell 8 is light in which the light having the frequency ν0 is absorbed by the alkali metal atom and disappears, while on the other hand, it is light having a peak at the frequency f clock . This light is converted into an electrical signal (transmitted light intensity signal S clock ) by the photodetector 9. At this time, the light that the photodetector 9 cannot follow is output as an intensity signal in the DC band. clock clock LI

[0026] The transmitted light intensity signal (resonance signal) S LI from the photodetector 9 is input to the discriminator 2 of the control circuit 1. The resonance signal S LI has a peak at the frequency f clock as described above, but has a low relative intensity of the peak and is a signal with a low SN ratio including noise outside the vicinity of the frequency f clock . The discriminator 2 generates an error signal S cnt from the control signal S LI from the built-in local oscillator 21 and the resonance signal S err and outputs it to the high-frequency oscillation circuit 3. The high-frequency oscillation circuit 3 is locked to the oscillation by the error signal S err to output a signal S OUTIt outputs the error signal S. err As a result of the change in capacitance, the oscillation frequency f of the high-frequency oscillator 31 changes. init For the frequency f init +Δf tune or f init -Δf tune Signal S OUT It outputs the signal S. OUT This is the signal S output by the frequency converter 41 in the frequency mixer 42. tune It is mixed with the frequency f clock Signal S of / 2 o1 This is the result. Signal S o1 The modulator 5 receives the control signal S from the local oscillator 21. cnt Based on this, the feedback signal S is converted. FB This is the result. The feedback signal S FB The Bias-T circuit 62 modulates the output light of the laser element 63 by superimposing it on the DC current from the current source 61.

[0027] (Startup operation of atomic oscillator) When the atomic oscillator 10 is synchronized (started up) from an asynchronous (unlocked) state, such as when it is started up or when it returns from a standby state, the control circuit 1 operates as follows. First, the high-frequency oscillation circuit 3 adjusts the oscillation frequency f of the high-frequency oscillator 31 without adjusting the frequency with the variable capacitor 32. init Signal S OUT Outputs the following: Oscillation frequency f init As shown in Figure 2A, f clock / 2±Δf tol It is within the range of f clock / 2±Δf tune_max It is outside the range. Note that in Figures 2A, 2B, and 2C, the position on the horizontal axis does not represent the actual frequency.

[0028] The control circuit 1 then receives the transmitted light intensity signal S LI While observing the intensity, the frequency converter 41 monitors its output signal S tune The frequency is swept to obtain the signal S that has the minimum intensity, i.e., the maximum absorption by the alkali metal atoms in gas cell 8. tune frequency f tune_cThe frequency converter 41 searches for the frequency f tune_c Fix it. As shown in Figure 2B, signal S tune By sweeping the frequency, the output signal S of the frequency mixer 42 o1 The frequency of is the oscillation frequency f of the high-frequency oscillator 31. init f clock Approaching / 2, f init +f tune_c (or f init -f tune_c ) but f clock / 2±Δf tune_max This falls within the range of [the specified range].

[0029] Signal S tune frequency f tune_c Once fixed, the next step is to capture the CPT resonance by sending an error signal S to the variable capacitor 32. err Inputting this signal will produce the output signal S of the high-frequency oscillation circuit 3. OUT The frequency is swept until absorption is maximized, i.e., the transmitted light intensity signal S LI The frequency at which the intensity is minimized is searched for and fixed. As shown in Figure 2C, the frequency sweep by the variable capacitor 32 controls the output signal S of the frequency mixer 42. o1 The frequency of this is Δf on the low-frequency side. tune Shift to f clock The result is / 2. In other words, the resonance frequencies of gas cell 8 are synchronized (locked), startup is complete, and a steady state is reached.

[0030] Output signal S of the atomic oscillator 10 (high-frequency oscillation circuit 3) in a steady state OUT The frequency is f init +f tune or f init -f tune And also, f clock / 2+f tune_c or f clock / 2-f tune_c Therefore, the output signal S OUT The frequency is the oscillation frequency f of the high-frequency oscillator 31. init ga f clock Sufficiently close to / 2 (f clock / 2±Δf tune_max Except in the case of the range of f clock This differs from / 2.

[0031] During subsequent startups, such as when returning from standby mode, the frequency converter 41 receives a high-frequency adjustment signal S. tune The sweep may be started from the frequency that was fixed just before entering the standby state. With this configuration, the resonance frequency can be synchronized early. Furthermore, if the standby time exceeds a predetermined time, a signal S is issued as an initialization signal. tune frequency f tune_c It may be configured to reset. Since the environment, such as the temperature of the atomic oscillator 10, may change during long waiting times, startup becomes more efficient.

[0032] In the atomic oscillator 10, the tolerance Δf of the oscillation frequency is controlled by the variable capacitor 32, the high-frequency adjustment oscillation circuit 4 (frequency converter 41, local oscillator 21), and the frequency mixer 42. tol The frequency f of the gas cell 8 synchronizes with the high-frequency oscillator 31, which has a large frequency. clock Acquire a signal of / 2. High-frequency adjustment signal S tune frequency f tune_c However, the tolerance Δf of the oscillation frequency of the high-frequency oscillator 31 is at most tol Since it is quite large, the variable width Δf is controlled by the variable capacitor 32. tune_max Even if the frequency f is small clock A signal of 2 / 2 can be obtained, and the Q value of the oscillation of the high-frequency oscillator 31 can be increased. On the other hand, since the oscillation frequency can be precisely adjusted by the variable capacitor 32, the high-frequency adjustment oscillation circuit 4 uses the local oscillator 21, which does not have high performance, to obtain a signal S for coarse adjustment. tune You just need to generate it.

[0033] In Figure 1, the output signal S of the high-frequency oscillation circuit 3 is used as the frequency standard signal in the atomic oscillator 10. OUT The output signal S of the frequency mixer 42 is taken out externally, but o1 It is also possible to extract the signal. However, in the atomic oscillator 10, since the oscillation frequency of the high-frequency oscillator 31 is the most stable frequency in the short term, it is preferable to output a signal that is closer to the output of the high-frequency oscillation circuit 3 to the outside.

[0034] The control circuit 1 is not limited to the above configuration, and for example, a circuit without a discriminator, as disclosed in International Publication No. 2021 / 049423, can be applied. Furthermore, the high-frequency adjustment oscillator circuit 4 does not reuse the local oscillator 21 (which also serves as the oscillator for the modulation signal of the modulator 5) of the discriminator 2. If the control circuit 1 has another oscillator, it may reuse that, or it may have its own built-in oscillator.

[0035] (Modified Version) In the atomic oscillator 10 according to the above embodiment, the output signal S of the high-frequency oscillation circuit 3 OUT The frequency f obtained by frequency conversion clock Signal S of / 2 o1 Modulated to form the feedback signal S FB It generates the signal S OUT The signal is modulated and then frequency-converted to produce the feedback signal S. FB You can also obtain it.

[0036] In other words, as shown in Figure 3, the atomic oscillator 10A according to a modified example of the first embodiment has a configuration in which the order of the frequency mixer 42 and the modulator 5 of the control circuit 1 is reversed compared to the atomic oscillator 10. In the atomic oscillator 10A, the output signal S of the high-frequency oscillation circuit 3 OUT The signal S is first modulated by modulator 5. o2 High-frequency adjustment signal S tune The signals are mixed in the frequency mixer 42 to perform frequency conversion, and the feedback signal S is generated. FB The frequency stabilization and startup operations of the atomic oscillator 10A are the same as those of the atomic oscillator 10 described above.

[0037] [Second Embodiment] The signal mixed by the frequency mixer may contain an image frequency signal (image signal). Light modulated by the feedback signal generated from such a signal contains noise and may interfere with the frequency stabilization operation of the atomic oscillator, so it is preferable to remove the image signal. Therefore, as shown in Figure 4, the atomic oscillator 10B according to the second embodiment of the present invention is provided in the control circuit 1B by inserting an image rejection filter 46 as an image signal removal circuit between the frequency mixer 42 and the modulator 5. The other configurations are the same as those of the atomic oscillator 10 according to the first embodiment shown in Figure 1.

[0038] The image removal filter 46 cuts (attenuates) the frequency band of the image frequency, and f clock This is a filter that transmits a frequency band of 2. The image removal filter 46 filters the signal S o1 The image signal is removed.

[0039] (Modified Version) In the control circuit 1B, the image rejection filter 46 only needs to be located on the output side of the frequency mixer 42. Therefore, as shown in Figure 5, the atomic oscillator 10C according to the modified version of the second embodiment is provided with the image rejection filter 46 and modulator 5 reversed compared to the atomic oscillator 10B, with the image rejection filter 46 located on the output side of the modulator 5 (between it and the Bias-T circuit 62). Also, as in the atomic oscillator 10A according to the modified version of the first embodiment (see Figure 3), the output signal S of the high-frequency oscillation circuit 3 OUT The image signal can also be removed from the signal that has been modulated by the modulator 5 and then frequency-converted by the frequency mixer 42. That is, as shown in Figure 6, the atomic oscillator 10D according to a modification of the second embodiment is equipped with an image removal filter 46 in the control circuit 1D, which is placed on the output side of the frequency mixer 42 (between it and the Bias-T circuit 62). The other configurations are the same as those of the atomic oscillator 10A.

[0040] The frequency stabilization and startup operations of the atomic oscillators 10B, 10C, and 10D are the same as those of the atomic oscillator 10 according to the first embodiment.

[0041] [Third Embodiment] The image signal removal circuit is not limited to a filter and can also be configured as follows. As shown in Figure 7, the atomic oscillator 10E according to the third embodiment of the present invention has a control circuit 1E which is provided with a 90-degree phase shifter 43, a frequency mixer 44, and a frequency adder 45 added to the control circuit 1 as an image signal removal circuit. The frequency mixer 44 is connected in parallel with the frequency mixer 42, and therefore the output signal S of the high-frequency oscillation circuit 3 OUT This signal is then input to frequency mixer 42 and frequency mixer 44. The outputs of frequency mixers 42 and 44 are then input to frequency adder 45, and its output is input to modulator 5. In addition, a high-frequency adjustment signal S is input. tuneHowever, the signal is input to the frequency mixer 42 and the 90-degree phase shifter 43, and the output of the 90-degree phase shifter 43 is input to the frequency mixer 44.

[0042] A 90 degree phase shifter (π / 2 phase shifter) 43 receives a signal S tune The phase is shifted by 90°. The frequency mixer 44 can have the same configuration as the frequency mixer 42, and the high-frequency signal S OUT The signal S is then transmitted via the 90-degree phase shifter 43. tune The signal S is mixed with the phase-shifted signal to perform frequency conversion. The frequency adder 45 processes the signal S output by the frequency mixer 42. o1 The signal output by the frequency mixer 44 is added to the signal output by the frequency mixer 44. By adding the two signals, each frequency-converted by signals with a phase difference of 90° from each other, the image signal is removed.

[0043] (Modified Version) As shown in the modified atomic oscillator 10A of the first embodiment (see Figure 3), the output signal S of the high-frequency oscillation circuit 3 OUT Even for a signal that has been modulated by modulator 5 and then frequency-converted by frequency mixer 42, the image signal can be removed using the same removal circuit as for atomic oscillator 10E. That is, as shown in Figure 8, the atomic oscillator 10F according to a modified example of the third embodiment removes the image signal from the signal S modulated by modulator 5. o2 The signals are input to frequency mixer 42 and frequency mixer 44, and their respective outputs are added together in frequency adder 45 to form the feedback signal S. FB The rest of the configuration is the same as that of atomic oscillator 10E.

[0044] The frequency stabilization and startup operations of the atomic oscillators 10E and 10F are the same as those of the atomic oscillator 10 according to the first embodiment.

[0045] While embodiments for implementing the atomic oscillator according to the present invention have been described above, the present invention is not limited to these embodiments, and various modifications are possible within the scope of the claims.

[0046] 10, 10A-10F Atomic Oscillator 1, 1A-1F Control Circuit 2 Discriminant 21 Local Oscillator 3 High-Frequency Oscillator Circuit 31 High-Frequency Oscillator 32 Variable Capacitor 4 High-Frequency Adjustment Oscillator Circuit 41 Frequency Converter 42, 44 Frequency Mixer 43 90-Degree Phase Shifter 45 Frequency Adder 46 Image Removal Filter (Removal Circuit) 5 Modulator 6 Laser Light Source 61 Current Source 62 Bias-T Circuit 63 Laser Element 7 Optical System 8 Gas Cell 9 Photodetector

Claims

1. An atomic oscillator that generates a resonant frequency by CPT resonance, comprising: a gas cell containing alkali metal atoms; a photodetector that detects light transmitted through the gas cell and converts it into an electrical signal; a control circuit equipped with a high-frequency oscillator that generates a feedback signal from the electrical signal and the signal output by the high-frequency oscillator; and a laser light source that outputs light modulated by the feedback signal and introduces it into the gas cell, wherein the control circuit further comprises: a variable capacitor that adjusts the oscillation frequency of the high-frequency oscillator using the signal output by the photodetector; a high-frequency adjustment oscillator circuit that outputs a high-frequency adjustment signal; and a frequency mixer that mixes the signal output by the high-frequency oscillator or a modulated signal of the signal with the high-frequency adjustment signal and outputs a signal at a frequency that synchronizes with the resonant frequency of the gas cell, wherein the maximum frequency that the high-frequency adjustment oscillator circuit can output is greater than the variable range of the oscillation frequency of the high-frequency oscillator by the variable capacitor.

2. The atomic oscillator according to claim 1, wherein the control circuit further comprises a local oscillator and a modulator that modulates an input signal based on a signal output by the local oscillator, and the high-frequency adjustment oscillator circuit comprises a frequency converter that converts the frequency of the signal output by the local oscillator to output the high-frequency adjustment signal.

3. The atomic oscillator according to claim 1 or claim 2, further comprising a removal circuit for removing an image signal from the signal output by the frequency mixer.

4. The atomic oscillator according to claim 1 or claim 2, wherein the high-frequency oscillator comprises a piezoelectric thin-film resonator.

5. The atomic oscillator according to claim 4, wherein the control circuit has an on-chip structure.

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